1. Field of the Invention
The present invention relates to a combustion-engined setting tool for driving fastening elements in a workpiece and including a combustion chamber for fuel, a piston guide, a drive piston displaceable in the piston guide, having a piston head and driven by expanding gases produced in the combustion chamber, and a device for returning the drive piston in its initial position as a result of pressure difference between chambers formed on opposite axial sides of the piston head.
2. Description of the Prior Art
Setting tools of the types described above can operate on gaseous or evaporated liquid fuels which are combusted in a combustion chamber and thereby drive a drive piston for driving fastening elements. After completion of a setting process, the drive piston should be returned to its initial position adjacent to the combustion chamber, so that the setting tool is ready for a next setting process.
German Publication DE 195 09 763 A1 discloses a combustion-engined setting tool having a drive piston displaceable in a piston guide and driven by propellant gases and which drives a fastening element in a constructional component with the piston shaft. The return of the drive piston in its initial position is effected by an elastomeric compression or tension spring.
The drawback of the setting tool disclosed in the German publication consists in that the drive piston at a certain state of wear does not return completely in its initial position, which leads to reduction of the available setting energy. On the other hand, particles of the elastomeric spring can become loose, interfering with the setting tool functions.
European Publication EP 0 056 989 A1 discloses a further combustion-engined tool with a piston displaceable in a piston guide and in which the drive piston is returned to its initial position by a pressure difference (or a differential pressure) between the environmental pressure that acts on the side of the drive piston remote from the combustion chamber, and the pressure created in the combustion chamber.
The drawback of the tool disclosed in the European Publication consists in that a faulty position of the piston can occur when, e.g., the friction of the drive piston increases as a result of contamination, and the piston does not return completely in its initial position, or when as a result of a too small difference between the environmental temperature and the power tool temperature, the pressure difference is not sufficient for a complete return of the drive piston in its initial position.
Accordingly, an object of the present invention is a setting tool in which a low-wear and reliable return of the drive piston in is initial position is possible.